Theres an almost psychopathic detachment to David Sumner, a lack of empathy so great that it sees him nonchalantly destroy a horde of Cornish rednecks before he proceeding gloat about it like hes just won a competition. The point of Peckinpahs masterpiece is to show how even a seemingly normal man can succumb to violent urges, with Sumner our relatable protagonist and way in to the film. Watch Straw Dogs again, though, and youll see that Sumner who gleefully antagonises everyone around him with his selfishness and air of superiority is a basket case from the very beginning. See also: Death Wish, and any number of other films where a regular guy suddenly goes postal.